An infrared detector called the “bolometer,” now well known in the art, operates on the principle that the electrical resistance of the bolometer material changes with respect to the bolometer temperature, which in turn changes in response to the quantity of absorbed incident infrared radiation. These characteristics can be exploited to measure incident infrared radiation on the bolometer by sensing the resulting change in its resistance. When used as an infrared detector, the bolometer is generally thermally isolated from its supporting substrate or surroundings to allow the absorbed incident infrared radiation to generate a temperature change in the bolometer material, and be less affected by substrate temperature.
Modern microbolometer structures were developed by the Honeywell Corporation. By way of background, certain prior art uncooled detectors and/or arrays, for example those manufactured by Honeywell, Inc., are described in U.S. Pat. Nos. 5,286,976, and 5,300,915, and 5,021,663, each of which is hereby incorporated by reference. These detectors include those uncooled microbolometer detectors which have a two-level microbridge configuration: the upper level and lower level form a cavity that sensitizes the bolometer to radiation of a particular range of wavelengths; the upper level forms a “microbridge” which includes a thermal sensing element; the lower level includes the read-out integrated circuitry and reflective material to form the cavity; the upper microbridge is supported by legs which thermally isolate the upper level from the lower level and which communicate electrical information therein and to the integrated circuitry.
A list of references related to the aforesaid structure may be found in U.S. Pat. No. 5,420,419. The aforesaid patent describes a two-dimensional array of closely spaced microbolometer detectors that are typically fabricated on a monolithic silicon substrate or integrated circuit. Commonly, each of these microbolometer detectors are fabricated on the substrate by way of what is commonly referred to as, a bridge structure which includes a temperature sensitive resistive element that is substantially thermally isolated from the substrate. This aforesaid microbolometer detector structure is often referred to as a “thermally-isolated microbolometer.” The resistive element, for example may be comprised of vanadium oxide material that absorbs infrared radiation. The constructed bridge structure provides good thermal isolation between the resistive element of each microbolometer detector and the silicon substrate. An exemplary microbolometer structure may dimensionally be in the order of approximately 25 microns by 25 microns.
In contrast, a microbolometer detector that is fabricated directly on the substrate, without the bridge structure, is herein referred to as a “thermally shorted microbolometer,” since the temperature of the substrate and/or package will directly affect it. Alternately, it may be regarded as a “heat sunk” pixel since it is shorted to the substrate.
“Microbolometer” is used herein to refer to both a “thermally-isolated” and a “thermally shorted” microbolometer. As used herein, a “microbolometer” refers to any bolometer type of construction in which a sensor exhibits a resistance change as a function of the observed radiation impinging thereon.
Microbolometer detector arrays may be used to sense a focal plane of incident radiation (typically infrared). Each microbolometer detector of an array may absorb any radiation incident thereon, resulting in a corresponding change in its temperature, which results in a corresponding change in its resistance. With each microbolometer functioning as a pixel, a two-dimensional image or picture representation of the incident infrared radiation may be generated by translating the changes in resistance of each microbolometer into a time-multiplexed electrical signal that can be displayed on a monitor or stored in a computer. The circuitry used to perform this translation is commonly known as the Read Out Integrated Circuit (ROIC), and is commonly fabricated as an integrated circuit on a silicon substrate. The microbolometer array may then be fabricated on top of the ROIC. The combination of the ROIC and microbolometer array is commonly known as a microbolometer infrared Focal Plane Array (FPA). Microbolometer focal plane arrays of 640.times.480 detectors are often employed within infrared cameras.
Individual microbolometers will have non-uniform responses to uniform incident infrared radiation, even when the bolometers are manufactured as part of a microbolometer FPA. This is due to small variations in the detectors' electrical and thermal properties as a result of the manufacturing process. These non-uniformities in the microbolometer response characteristics must be corrected to produce an electrical signal with adequate signal-to-noise ratio for image processing and display.
Under the conditions where a uniform electric signal bias source and incident infrared radiation are applied to an array of microbolometer detectors, differences in detector response will occur. This is commonly referred to as spatial non-uniformity, and is due to the variations in a number of critical performance characteristics of the microbolometer detectors. This is a natural result of the microbolometer fabrication process. The characteristics contributing to spatial non-uniformity include the infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and thermal conductivity of the individual detectors.
The magnitude of the response non-uniformity can be substantially larger than the magnitude of the actual response due to the incident infrared radiation. The resulting ROIC output signal is difficult to process, as it requires system interface electronics having a very high dynamic range. In order to achieve an output signal dominated by the level of incident infrared radiation, processing to correct for detector non-uniformity is required.
Methods for implementing an ROIC for microbolometer arrays have used an architecture wherein the resistance of each microbolometer is sensed by applying a uniform electric signal source, e.g., voltage or current sources, and a resistive load to the microbolometer element. The current resulting from the applied voltage is integrated over time by an amplifier to produce an output voltage level proportional to the value of the integrated current. The output voltage is then multiplexed to the signal acquisition system.
One source of error in microbolometer focal plane arrays is the variation in “responsivity” of each microbolometer with changes in temperature. This is because each microbolometer is a temperature sensitive resistor having a sensitivity that varies due to minute variances in the manufacturing process from one microbolometer to another. The temperature of a microbolometer is affected by the bias current flowing there through, since that current inherently warms the microbolometer due to the power dissipated. Heat is also transferred to the microbolometer through the focal plane array's substrate. Heat is also transferred to the microbolometer by infrared radiation incident on the microbolometer.
Infrared focal plane arrays (IRFPAs) such as microbolometer arrays typically exhibit pixel-to-pixel variations in responsivity and offset. These variations can contribute to persistent artifacts in the images that are known as “fixed pattern noise.” Fixed pattern noise is identified by the presence of a persistent pattern or artifact that exists in the resulting imagery in a repeatable, long-term pattern. In contrast to temporal noise, fixed pattern noise persists from frame to frame for long periods. Because of this, it cannot be removed by averaging data from multiple frames.
Another factor that can contribute to fixed pattern noise is the existence of “stray” infrared (IR) radiation. The stray radiation includes any IR radiation that is incident on the pixel but that did not come from the scene of interest (i.e., that did not come from the IR energy focused on the IRFPA by the lens). Potential sources of stray IR radiation include the IRFPA's vacuum package, the barrel of the lens, the lens itself (if its emissivity is greater than zero), the camera housing and electronics, etc.
The fixed pattern noise that can occur in IR cameras can have almost any shape or appearance. For example, some sensors have exhibited fixed pattern noise that looks like a vertical stripe pattern overlaid onto the desired scene. Others have a more grid-like appearance which can be likened to looking through a screen door; i.e., you can see the scene, but there is a grid-like pattern overlaid on it. Still other sensors exhibit “spots,” “blotches” or “clouds” in various areas of the image. FIGS. 1A and 1B show examples of fixed pattern noise 300 on an image, as compared to FIGS. 1C and 1D where the majority of fixed pattern noise has been removed from the image.
Typical IR camera systems apply “two-point” gain and offset corrections to correct for pixel to pixel non-uniformity and minimize fixed pattern artifacts. The offset data is obtained in “real time” by periodically actuating a shutter that is included as a part of the camera system. The gain data is effectively the inverse of the responsivity data. The gain data is generally obtained during a calibration process at the factory and is stored in nonvolatile memory on board the camera. However, the two-point correction method usually only works well when the IRFPA has relatively small pixel-to-pixel variations in responsivity and offset, and when the IRFPA is temperature stabilized. Camera systems that utilize a shutter that is located between the IRFPA and the lens (which is a common configuration) are particularly susceptible to fixed pattern artifacts caused by pixel-to-pixel variations in responsivity.